1. Field of the Invention
This invention is in the field of sulfur purification. More particularly, it relates to a process for removing hydrogen sulfide contamination from sulfur.
2. Prior Art
Molten sulfur, when obtained as a byproduct of crude oil refining, contains substantial amounts of hydrogen sulfide as it comes out of the Claus reactors and goes into the storage pits. In these reservoirs the amount of dissolved hydrogen sulfide generally ranges between 20 and 1000 ppm and especially between 50 and 100 ppm. In periods of unstable operation, they may contain even more hydrogen sulfide such as 2000 ppm or more. This creates odor problems and personnel hazards, not only around these storage tanks and at the sulfur delivery points but also in and around the tank trucks and railroad tank cars used to transport this product.
With more than 40 ppm hydrogen sulfide in the liquid, the vapor in equilibrium with the liquid is highly combustible. But even at levels as low as 1 to 2 ppm hydrogen sulfide in the liquid, the corresponding vapors are still toxic to human life. Fatalities and near fatal accidents occur with disquieting frequency.
The most widely used modern sulfur purification units are based on the French SNPA process, where the hydrogen sulfide is removed by sparging air through the molten sulfur. Under the French design conditions the major drawback of this process is that it is slow; in addition, even a standard three-stage version does not reduce the hydrogen sulfide level in the final product to less than 5 ppm. Although this is adequate to eliminate explosion hazards, it is still insufficient to ensure operational safety.
Water washing of molten sulfur has also been used to reduce hydrogen sulfide levels. U.S. Pat. No. 4,002,728 of DeMay describes such a process. This process is marginally better than the SNPA process. It has the failing of requiring multiple stages and high water use levels to reduce hydrogen sulfide below about 1 ppm. In fact, with 2 parts by volume of water per part of sulfur in a single stage generally from 10 to 15 ppm of hydrogen sulfide remains. This patent suggests mixing and heating the sulfur-water mixture with steam but in general teaches against such vapor phase stripping.
Concurrent contact of atomized sulfur and steam at 120.degree. C.-160.degree. C. is shown to remove hydrogen sulfide to from 7 to 33 ppm levels in Japanese Kokai Sho 49-108312 of Nippon Kihatsuya Co., Ltd., while Kokai Sho 48-123653 of Nippon Sekiyu Co., Ltd. discloses that injecting small amounts (0.15 kg/kg . hr) of steam into molten sulfur for 2 to 4 hours can reduce hydrogen sulfide levels to about 10 ppm.
Three other patent references which are of interest to the present process are U.S. Pat. No. 3,273,966 of Bowling, et al, which discloses the removal of CS.sub.2, iron and sulfur chloride from sulfur produced as a byproduct of CCl.sub.4 production, by means of contact with heated water; U.S. Pat. No. 3,647,398 of Goldberger which uses a countercurrent contacting of crude sulfur with hot water to remove gange therefrom; and U.S. Pat. No. 4,131,437 where hydrogen sulfide is removed from liquid sulfur by contact with a carrier gas.
It is the object of this invention to provide an improved process for very thoroughly removing hydrogen sulfide from elemental sulfur.